I'm 20 right now, and I'm at the point in my life where maybe I should just 'grow up' as well and stop MMOs. But I think of this: FFXI was some of the BEST times I've had in my life. For a while I was really addicted, in the truest sense, but I broke the habit and ffxi became kind of recreational, and that just doesn't really work well with FFXI so I ended up quitting.
I made so many AWESOME friends in FFXI, who says that it takes away from your social life? When I quit I regained a lot of boring free time... time that I wanted to spend online with my FFXI friends -and why not? Are people you enjoy the company of, share common hobbies with, laugh, gossip, and converse with less of 'friends' because you use a digital medium instead of going to the bowling alley? Better yet, why not have BOTH kinds of friends and divide your life evenly?
I think as long as you are responsible with your game playing, and don't do it too much (just like IRL you shouldn't spend too much time out with your friends boozing or whatever...) then it's perfectly fine to continue playing MMOs. ESPECIALLY if you prefer MMO social life over IRL social life.
Also, I honestly believe the internet will be the commonplace way friends will spend time with each other in the future anyway...
I am also 20 (this seems to be a trend) and while I met a couple amazing friends on FFXI (which I never saw happening) I don't know if playing a videogame would constitute as having some of the best times of my life. While technology is a great thing and I enjoy playing videogames, it should never replace physical social interactions. No matter what anyone says, it is not the healthiest form of communication.
My father also plays videogames, so I don't think playing them means that one lacks maturity, so don't worry about that. Out of all the things that contribute to someone being mature, I wouldn't count "videogames" as a huge deciding factor. It's definitely not going to stop me from playing. Playing a game with a good storyline can be somewhat compared to watching a good movie/television show or reading a good book. It can definitely be stimulating, but even sitting around reading books all day is not the healthiest idea.
The reason people gain tons of boring free time after they quit is because they don't know how to take up other hobbies. It's really not that hard to go out and meet new people once you quit a videogame. I would quit for months on end because I lacked the time to play due to boyfriends, sports, school, etc. When I had free time I chose to play because videogaming is fun and so are MMO's, not because I felt that the social life was better than IRL because it most certainly is not. Maybe they can be placed on the same level, but human beings were put on this earth to interact with each other physically to some degree. There is more to social interaction than just talking; there is actual physical chemistry involved and emotions will never be as fully displayed through a technological medium as they are through actual people.
And to add to the health comments, sitting in front of a computer all day with a headset on = not good. People need sunlight and exercise on a daily basis. It's important to keep all of these things in mind when it comes to any hobby.
Like I said, I have also made some amazing friends through FFXI, and I think balancing social interaction between the internet and real life can be fine, but letting an MMO completely dominate your social life; there's really no good argument for that that I can think of.